Effect of particle size and partial replacement of alfalfa hay by soya bean hulls on nutrient intake, total tract digestibility and rumen degradability of diets by Holstein steers

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Soya bean hulls (SH) have been used as a replacement for forages in diet of dairy cows and beef cattle because of their high energy and fibre content. However, in contrast to forage fibre, the SH fibre is highly digestible making it a poor source of effective fibre (Behgar et al., 2011). Hsu et al. (1987) showed that the digestion of SH was higher in situ than in vivo, which was attributed to the short retention time of soya bean hulls in the rumen. Similarly, Nakamura and Owen (1989) observed that the passage rate of ytterbium‐labelled SH was two times greater than labelled alfalfa hay (AH). Rapid ruminal passage rate of SH has been mainly related to their small particle size and high specific gravity in the rumen (Titgemeyer, 2000). Increased dry matter intake (DMI) has been also suggested to have a role in the high passage rate of SH from rumen (Mulligan et al., 2001); however, increasing the level of DMI of steers did not completely explain high passage rate of SH and its effects on ruminal fibre digestibility (Woods et al., 1999).
It has been well established that replacement of a portion of hay of diet with SH decreased the mean particle size of diet (Weidner and Grant, 1994b; Slater et al., 2000) and also resulted in reduced rumination time (Weidner and Grant, 1994b) and ruminal mat consistency (Weidner and Grant, 1994a).
Addition of long hay to diets containing SH has been shown to interact with SH in the rumen, resulting in decreased passage rate of SH, increased rumination time and increased extent of ruminal digestion (Weidner and Grant, 1994a,b). Behgar et al. (2011) found that increasing diet forage particle size can prevent milk fat depression in dairy cows fed SH‐based diets. However, in this study, faecal pH decreased in cows fed coarse diets containing SH compared to those fed fine diets, suggesting that larger particle size could shift the digestion of SH from rumen to the lower gut.
Although a large number of studies have studied SH as a replacement for forage or grain in cattle diets, there is very limited information on the effect of particle size of diets on the utilization of SH by cattle. Therefore, the objective of the present study was to evaluate the impact of AH particle size in SH‐based diets on total tract digestion, in situ degradation of diets and rumen condition in Holstein steers fed at restricted level of intake.
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